Electrical heating apparatus



May 13, 1947. J. Di GRIFFIN ELECTRICAL HEATING APPARATUS Original FiledMarch 8, 1943 Patented May 13, 1947 I 1 ELECTRICAL HEATING APPARATUSJoseph Darius Griflin, Johannesburg, Transvaal, Union of South AfricaOriginal application March 8, 1943, Serial N 0. 478,375. Divided andthis application May 15, 1945, Serial No. 593,882. In the Union of SouthAfrica, November 21, 1942 13 Claims.

This application is a division of my application Serial No. 478,375,filed on March 8, 1943.

This invention relates to salt bath furnaces.

Salt bath furnaces have been found advantageous for heating the cuttingends of rock drill tools; and furnaces have been devised in which suchtools are arranged in a substantially vertical position with theircutting ends downward and dipped below the surface of the bath materialonly so far as the ends require to. be heated for forging or temperingthem as the case may be, The invention consists in improvements in thiskind of furnace having as their object increase of the throughput of thefurnace and reduction of labour in ensuring that the proper depth ofimmersion is attained.

For instance, in bath furnaces heretobefore devised for heating thelower ends of rock drill tools andother rod-like bodies in the mannerdescribed, the bodies had to. be suspended from insulating supportingmeans above the bath and kept from contact with one another; and themeans to ensure the immersion of the lower ends only to thepredetermined extent has been somewhat cumbersome and has delayed thetransfer of the drills to the next operation. This diiiiculty isovercome according to the invention by providing, in the bath well, asupport positioned in the well above the bottom of the well and belowthe working level of the surface of the bath material and forming a restfor bodies placed in the bath material, said rest being sufficientlynear to said working level to limit the immersion of the ends to thatrequisite for the object of the heating, viz: the aforesaid forging ortempering. The bath level is kept at a substantially uniform heightabove the support; and the correctness of the level is easily checked byobserving the extent of the heated. length of the drills as they arewithdrawn from the furnace.

A further improvement useful in connection with the heating of rockdrill tools is that the well is provided with a cover having aperturingno larger than is convenient for the drills to pass through into thebath; thus conserving the heat of the bath.

When operating at high temperature appreciably in excess of the fusingtemperature of the salts used it is desirable to get rid of the fumesthat arise from the bath and the invention provides an arrangement fordoing this in an effective manner that prevents the great loss of heatwhen the well cavity is directly ventilated so that cold air is drawninto it. For this purpose the bath well is substantially enclosed exceptfor the above-mentioned cover apertu-ring; and there is provided anexhaust duct having its inlet mouth positioned to collect gas tending topass from the interior of the well to the open air, through saidaperturing.

It is necessary toprovide some means of steadying the upper portions oflong drills when their lower ends rest on the support. There is providedfor this purpose a rest positioned at a higher elevation than thefurnace cover and. providing, in plan, a gap having an open end and aclosed end, the latter displaced to one side of the cover slot andspaced from the open end in the direction of such displacement.

The invention is illustrated in the accompanying drawings in whichFigure I is a perspective view of a rock drill heating furnace withparts broken away to show the interior.

Figure II is a vertical section transverse to the length of the furnace.

The bath structure shown in Figures I and II comprise walls 2 and a base3 of heat resisting material, enclosing a bath well '4. In the reg ularuse of the furnace said well contains a mass 5. of molten salt with itssurface at about the level a.-a. For the purpose of heatingsimultaneously a number of rock drill tools 6 arranged in a row, it isconvenient to. make the well considerably longer than its Width.

A cover I closes in the well cavity to a large extent and so preventsloss of heat from the bath as well as shielding the operator fromradiated heat. The drill tools 6 require to be heated only at theircutting ends 8. The cover is accordingly provided with a longitudinalslot 9 suitable to receive a row of drill tools with the lowered endssubmerged in the bath and no wider than is reasonably necessary for thatpurpose; or there may be a row of holes equivalent to a slot.

The bath heating elements are indicated by Ill and II respectively, andare shown positioned one above the other; the element I0 resting on thebase. 3. of the bath well with its terminals l2, l3 extending up throughslots M. in the ends of the bath structure. Said elements are about aslong as the bath well.

In the combination of elements here shown, the elements llland H arearranged to heat the bath electrcdically after the bath material hasbeen melted, but element It] may be arranged to melt the bath materialby resistance heating.

Element I I serves as a support for the tools 6, a datum level fordetermining the level at which the bath surface should be maintained,

and a shield to prevent accumulation of foreign matter in the bottom ofthe well.

The element H is a simple plate that rests removably and in a horizontalposition on shoulders I5 provided by the walls 2 of the bath structure.In the use of the bath, the amount of the bath material is so regulatedthat it submerges plate ll. Element ID being at the bottom of the wellis also totally submerged, so that neither electrode intersects anair-salt interface.

The tool ends 8 rest on the plate H and the depth of molten salt abovethe upper surface of said plate is regulated according to the length ofthe lower end of the tools that is to be directly heated by immersion inthe salt, say one half inch for hardening or one and a half to twoinches for forging.

A result of supporting the tools on the upper side of plate ll is thatthey are well out of the electric circuit. This gives the importantadvantage that there is no material difierence of electrical potentialbetween the several tools of a batch, which would necessitate theirbeing insulated from one another. A second important advantage is thatvariation of the number of tools inserted into the bath does notinterfere with the electrodic heating.

In salt baths in which the electrodes project above the surface leveland intersect the air-salt interface, slight changes of the level of thebath alter the electrode area in contact with the salt, and consequentlythe magnitude of th resistance to the electrodic heating current and thetemperature of the bath. This instability is serious enough to make itcustomary to install pyrometer control of the current, necessitatingcontrol devices which form a considerable percentage of the total costof the furnace. In the present case, the fact that the bath level iswell above the electrodes avoids any variation of the effectiveelectrodic area; with the result that no expensive temperature controldevice is necessary when the line voltage is reasonably constant.

One of the difficulties of heating ferrous articles in salt bathfurnaces heretofore has been due to scale detaching itself from thearticles and accumulating in the bottom of the bath well. The plate I lconstitutes a shield for the bath well and itself receives the depositof scale, which can readily be scraped up from time to time for removal.This shielding function necessitates some channel by which the saltabove and below the plate is kept in free communication; and there isshown for this purpose grooving [6 in the upper surface of the shouldersleading to longitudinal grooves ll beyond the long edges of plate ll.Said grooves l1 form gutters into which foreign matter that may spilloff the edges of plate II is received without tending to pass into thegrooving I6 and from which grooves such matter can readily be scrapedout.

As has been pointed out, the weight of the drills 6 is carried by theplate I l; and the length of the heated zone of the cutting ends isdetermined by the depth to which H is submerged in the molten bathmaterial; consequently the elaborate drill supporting means heretoforeused and comprising means adjustable according to the length of eachdrill, can be done away with. All that remains to be provided is asimple means of engaging the upper portions of the drills to preventtheir toppling over. For this purpose gapped rests are provided at ahigher elevation than the furnace cover and to one side thereof,

(ill

and such that upon the drills being tilted somewhat from the verticaltheir upper portions rest stably by gravity in said rests. Referring tothe drawings a pair of posts [8 is erected at the ends of the furnacestructure for the purpose of supporting a pair of racks I9, l9a. Eachrack consists of a horizontal rack bar 29 and 20a having at its endslugs 2i, 21a, for attaching it to the posts. The posts are slotted at 22to enable the racks to be secured at various heights accordin to theaverage length of the drills under treatment. Each bar is provided witha series of horizontally extending fingers 23, 23a providing betweenthem the recesses 24, 2411 into which the drill stems are passed bytilting them. To cause the drills to be inclined when within saidrecesses, the lugs Zia of the upper rack l9a are of such length as toset the bar 20a back from the slot 9 in plan; and the lugs 2 I a arelonger than the lugs 2! so as to set the bar 20 not so far back, inaccordance with the inclination of the drills. The two racks areprovided to keep the drills parallel with one another in the plane ofthe slot 9; but if the slot 9 is replaced by an equivalent series ofholes, one of the racks may be omitted.

One of the difficulties attending the operation of salt bath furnacesand particularly drill heating furnaces in the past has been concernedwith the disposal of the fumes that arise from the hot salt. Theventilating arrangements heretofore used have been such as to set up anair current through the well; but it has been found that the resultingloss of heat seriously diminishes the efiiciency of the furnace.

To avoid this loss, the bath well is kept closed except for the slot 9.Hot gas and fumes accordingly tend to escape through said slot; but theyare collected and prevented from passing into the open air by an exhaustduct comprising a series of holes 25 extending from one side wall 26 ofthe slot 9 and through the body of the cover 1 to the edge 2'! of thecover where they join flue channelling 28 formed in the furnacestructure. Said channelling leads to a chimney 29 which creates a draft,through the holes 25 and channelling 28, and so causes the fumes to bewithdrawn continually from the bath. The exhaust duct may be also placedimmediately above and in alignment with the longitudinal slot.

I claim:

1. An electrically heated salt bath furnace suitable for heating theends of steel bodies, comprising a well for the bath material, a supportpositioned in the well above the bottom of the well and below theworking level of the surface of the bath material and forming a rest forbodies placed in the bath material, said rest being sufficiently near tosaid working level to limit the immersion of the ends to that requisitefor the object of the heating.

2. The furnace claimed in claim 1, comprising a heating element shapedand arranged to form the support.

3. The salt bath furnace claimed in claim 1, in which the well isprovided with a cover adapted to remain in position while rod-likebodies are inserted into and withdrawn from the furnace, said coverproviding aperturing suitable for the lower endsof rod-like bodies to bepassed through it but of substantially less horizontal area than thehorizontal area of the bath surface, the well being substantiallyclosed-in except for this aperturing, and comprising an exhaust ducthaving its inlet mouth positioned to collect gas tending to pass to theopen air through said aperturing,

said exhaust duct comprising means to set up a ventilating current fromits intake end next to the cover aperture to its discharge end, whichlatter is positioned to discharge the current clear of the furnace.

4. The salt bath furnace as claimed in claim 1, and suitable for heatingthe ends of rod-like bodies such as rock drill tools, comprising meanselevated above the Well for loosely engaging the upper ends of rod-likebodies and thereby in conjunction with the rest to hold such bodiesstably in a position that is sufficiently vertical to avoid materialextension of the aforesaid limitation of the immersion of their lowerends.

5. The salt bath furnace as claimed in claim 1, and suitable for heatingthe ends of rod-like bodies such as rock drill tools, comprising meanselevated above the well for loosely engaging the upper ends of rod-likebodies and thereby in conjunction with the rest to hold such bodiesstably in a position that is sufficiently vertical to avoid materialextension of the aforesaid limitation of the immersion of their lowerends; said elevated means providing in plan a gap having an open end anda closed end, the latter being displaced to one side of the cover:aperturing and spaced from the open end in the direction of saiddisplacement, so that stable gravitational support of the elongatedbodies is obtained by the lower ends resting on the support and theirupper ends leaning into the gap.

6. An electrically heated bath comprising a well for bath material andelectrical heating means comprising a resistance element and anotherelectrically conducting element arranged in said well, in which theresistance element is arranged in the lower part of the well and inwhich the other element is a plate positioned horizontally between saidresistance element and the mouth of the well so as to form a rest forarticles dipped in the bath material and to shield the lower part of thewell, and comprising means for maintaining communication between liquidbath material above and below the plate.

7. An electric heating bath comprising a well containing an electrolyte,a pair of superposed electrical conductors in said bath, the lowerconductor providing ohmic resistance for the melting of the electrolyte,and the upper conductor serving as a support for articles immersed inthe electrolyte and to be heated.

8. An electric heating apparatus comprising a vessel adapted to containan electrolyte, a pair of superposed electrodes in said vessel and belowthe normal liquid level of the electrolyte, the upper electrode beingadapted to support the lower ends of substantially vertically disposedrod-like articles immersed in said vessel and to be heated, and to limitthe extent to which the lower ends of the articles may be immersed inthe bath.

9. An electric heating apparatus comprising a vessel adapted to containan electrolyte, a pair of superposed electrodes in said vessel and belowthe normal liquid level of the electrolyte, the upper conductor beingadapted to support the lower ends of substantially vertically disposedrod-like articles inserted in said vessel and to be heated, and to limitthe extent to which the lower ends of said articles may be inserted inthe bath, a cover on said vessel having a slot through which thearticles to be heated may be inserted, and a rack for holding saidarticles in substantially vertical positions.

10. A bath comprising a well for containing molten bath material, acover for the well providing an aperture of small area compared with thehorizontal area of the well for the insertion therethrough of the lowerends of rod-like articles to be heated in the bath, and a pair ofsuperposed electrodes in the well below the surface of the bath, theupper of said electrodes serving as a support for limiting the extent towhich said articles may extend into the well.

11. An apparatus for electrically heating the ends of rod-like articles,comprising a well for bath material, electrical means within the wellfor heating the bath and including a horizontally disposed electrodeserving as a support for the lower ends of said articles and limitingthe extent to which said articles may be inserted in the bath, a coverfor said well having a slot through which the lower ends of the articlesmay be inserted in a row, and a rack above said cover for engaging theupper portion of said articles to space them and hold them insubstantially vertical positions.

12. An apparatus for electrically heating the ends of rod-like articles,including a well adapted to contain molten bath material, superposedelectrodes within the well and beneath the surface of the bath, theupper of said electrodes serving as a support for said articles, saidwell having a cover provided with a slot through which a row of saidarticles may be inserted into the bath to rest upon said upper electrodeand a rack above said cover for holding said articles in spacedsubstantially vertical positions.

13. An electrically heated bath comprising a well for bath material andelectrical heating means comprising a pair of electrical conductingelements within the bath, one of said elements bein in the form of aplate positioned horizontally and serving as a support fo articlesdipped into the bath and providing communication between the liquid bathabove and below the plate, and means above said well for engaging theupper parts of the articles to be heated and for holding them insubstantially vertical positions.

JOSEPH DARIUS GRIFFIN.

REFERENCES CITED UNITED STATES PATENTS Name Date Grifiin July 8, 1941Number

